In digital broadcasting receivers before correct receiving of transmitted data it is necessary to lock the system, i.e. the receiving device, to the correct frequency and the phase of the carrier of the distinct channel or signal to be received. Dependent on the particular transmission mode—via satellite, cable, or terrestrial transmission—certain modulations of the signals are employed, for instance QPSK, QAM, COFDM, respectively, for the aforementioned transmission modes.
For locking conventional receiving devices to the correct phase and frequency of the carrier signal, a phase and frequency detector is employed. Usually, the output of the detector is fed back to a phase de-rotator or frequency shifter within a phase locked loop (PLL). The phase locked loop achieves as a feedback loop successive corrections of the frequency and the phase of the receiving device until the system is locked to the correct frequency in phase of the signal to be received and evaluated.
When the system is not locked to the correct frequency and a frequency offset or frequency difference between the system frequency and the frequency of the carrier signal exceeds some limit, the phase error or phase difference between the system and the carrier signal changes in time periodically with a time average of the phase error or phase difference vanishing or being zero. A vanishing time average in the phase difference or phase error cannot give an indication for the PLL in which direction the system frequency has to be corrected to lock to the correct carrier frequency.
It is therefore an object of the present invention to provide a carrier recovery means in particular for a channel decoding unit and/or a digital demodulating unit particularly provided in a digital broadcasting receiver with an improved locking behavior which is capable of locking to the correct phase and frequency of a carrier signal in a reliable manner and in a particular short time.
This particular object is achieved by a carrier recovery means according to the present invention with the features of claim 1. Preferred and advantageous embodiments of the inventive carrier recovery means are within the scope of the dependent subclaims.